J . Am. Chem. SOC.1992, 114, 9869-9877 and Ni4a-c are indeed shifted to lower energy compared to NiTPP and NiM, and that Ni3 and Ni4a-c behave more like the very nonplanar porphyrins NiZb and NiZc. However, the magnitudes of the shifts in the absorption spectra of Ni3 and Ni4a-c are quite different. This may be because the porphyrins are distorted from planarity in different ways and/or to different degrees. Alternatively, it may be because the porphyrins have substituents with different electron-donating characteristics. INDO/CI calculations show that the correct core conformation and substituents are necessary to reproduce the trends in optical spectra of Nila, Nilb, Ni2a, and NiZb.3s Raman spectra of Ni3 and Ni4a-c also show quite clearly that these porphyrins exist in nonplanar conformations in solution. As Raman spectroscopyhas previously been used to show that NiOEP is present as a mixture of both planar and nonplanar conformations in solution,43we attempted to determine if Ni3 and Ni4a-c exist in multiple conformations in solution. Our results to date are inconclusive, but these studies are c o n t i n ~ i n g . ~ ~ Conclusions Dodecaphenylporphyrin(3) and the dodecaalkylporphyrins4a-c provide a unique opportunity to study many facets of porphyrin nonplanarity. We have investigated several aspects of the syntheses, structures, and spectroscopic properties of these por(44) Sparks, L. D., Shelnutt, J. A.; Medforth, C. J.; Smith, K. M. Unpublished results.
9869
phyrins. Three of the key results obtained from our studies are as follows: (1) A very nonplanar saddle distortion is seen in the crystal structure of 3, and a very nonplanar ruffled distortion is seen in the crystal structure of Ni4c. (2) These nonplanar distortions are correctly predicted by molecular mechanics calculations using a force field for nickel(I1) porphyrins. (3) Variable temperature N M R studies indicate that the macrocycle conformations of 3 and N i C in solution are similar to those seen in the crystalline state. Acknowledgment. Work performed at the University of California was supported by the National Science Foundation (CHE-90-0138 1, K.M.S.) and the Deutsche Forschungsgemeinschaft (Se 543/1-1, M.O.S.). Work performed at Sandia National Laboratories was supported by the U.S. Department of Energy (DE-AC04-76DP00789, J.A.S.). C.J.M. thanks the Fulbright Commission for the award of a travel scholarship. C.J.M. and L.D.S. thank the Associated Western Universities, Inc., for fellowships. Supplementary Material Available: Details of the crystal structure determinations of 3 and Ni4c including lists of atomic coordinates, anisotropic thermal parameters, H-atom coordinates, bond lengths, bond angles, torsion angles, and least-squares planes (29 pages); observed and calculated structure factors for 3 and Ni4c (37 pages). Ordering information is given on any current masthead page.
Synthesis and Characterization of a Superoxo Complex of the Dicobalt Cofacial Diporphyrin [ (p-02)Co2(DPB)( 1,5-diphenylimida~ole)~] [PF6], the Structure of the Parent Dicobalt Diporphyrin Co2(DPB), and a New Synthesis of the Free-Base Cofacial Diporphyrin H4(DPB) James P. Collman,**tJames E. Hutchison,t Michel Angel Lopez,*Alain Tabard,* Roger Guilard,: Won K. Seek,§ James A. Ibers,s and Maurice L'Herl Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305, UniversitC de Bourgogne, Laboratoire de SynthZse et d'ElectrosynthPse OrganomCtalliques associC au CNRS (URA 33), Facult? des Sciences "Gabriel",21100 Dijon Cedex, France, Department of Chemistry, Northwestern University, Evanston, Illinois 60208-31 13, and UniversitC de Bretagne Occidentale URA CNRS 322, Facult; des Sciences, 6 Avenue Victor Le Gorgeu, 29287 Brest Cedex, France. Received April 8, 1992 Abstract: Chemical oxidation of the dicobalt cofacial diporphyrin Co2"/"(DPB), followed by exposure to dioxygen affords the bridged superoxo complex [(p-02)Co2(DPB)][PF,]. This r-superoxo complex has been implicated in possible mechanisms of four-electron dioxygen reduction by dicobalt cofacial diporphyrins but had not been isolated and fully-Characterizedpreviously. Although the superoxo complex is unstable with respect to loss of dioxygen, addition of 2 equiv of 1,5-diphenylimidazoleyields [( ~ - 0 2 ) C o p 1 ' ( D P B1,5-diphenylimidazole),1 )( [PF,], a psuperoxo complex that is stable toward loss of dioxygen. Analytically pure samples of the latter complex have been prepared, and their spectral and electrochemical properties are described. The crystal structure of the parent dicobalt cofacial diporphyrin Co2"/"(DPB) is reported. The Co-Co distance (3.726 (1) A) and other structural features are compared to those of Co2(mF4) and of other known metallo-DPB structures. A new, improved total synthesis of the free-base porphyrin H,(DPB) is presented. The combination of new reaction sequences, increased reaction scales, and improved product yields allows for large scale synthesis (gram quantities) of the free-base porphyrin needed to develop fully the coordination chemistry of the cofacial metallodiporphyrins. The development of electrode materials that catalyze the four-electron reduction of dioxygen at or near the thermodynamic reduction potential (1.23 V vs NHE') is important for fuel-cell technology and has been the subject of intense study. Although 'Stanford University. Universitt de Bourgogne. 5 Northwestern University. Universitt de Bretagne Occidentale.
*
metallic platinum is an efficient fuel-cell cathode, its cost has prohibited routine use. Through the use of low loadings of platinum in solid polymer electrolytes, more economical, platinum-based fuel cells are presently being developed.2 Still, there (1 ) Abbreviations: DPBe = diporphyrinato biphenylene tetraanion, DPAC = diporphyrinato anthracene tetraanion, DPIm = 1,S-diphenylimidazole, E X = edge-plane graphite, FeCp, = ferrocene, N H E = normal hydrogen electrcde, DMSO = dimethyl sulfoxide, T H F = tetrahydrofuran.
0002-7863/92/ 15 14-9869%03.00/0 0 1992 American Chemical Society
9870 J. Am. Chem. SOC.,Vol. 114, No. 25, 1992
Collman et ai.
&
H-
P ( + co ;/
\
J
Figure 1. Cofacial dicobalt diporphyrins discussed in this paper: C o 3 ( R F 4 ) (upper left), Co,(DPB) (upper right), and Co,(DPA) (lower).
is interest in developing new electrode materials for dioxygen reduction and in understanding the mechanisms of multielectron transfers involved in dioxygen reduction. Co,( FTF4)334and other cofacial metallodiporphyrins4-' (Figure 1) have been developed by several groups and have been shown to catalyze the fourelectron reduction of dioxygen at moderately high potentials when the catalyst is adsorbed on edge-plane graphite that is in contact with dioxygenated, acidic aqueous solutions. Although considerable effort has been made to understand the mechanism of four-electron dioxygen reduction by cofacial metallodiporphyrins, the details of the catalysis remain unclear. Initially it was believed that C O ~ ~ / " ( F T Fwas ~ ) the active catalyst and that dioxygen reacted with it to form the peroxo complex (p-02)Co2(FTF4), in which O2 was bridging the two metals.3 It was p r o m that subsequent protonations and electron additions led to 0-0 bond cleavage and formation of water. Several variations of this mechanism have been suggested as the result of other s t ~ d i e s . ~ , ~ ~ ~ In nonaqueous solvents, Co211/11( FTF4) is oxidized by one electron to produce either a mixed-valent cation (CO,~~/II~) or a porphyrin ring-centered radical cation, depending upon the solvent.12 Whatever the nature of the cation, in benzonitrile its ~~
~
~~~~~~~~
~~~
(2) (a) Ticianelli, E. A.; Derouin, C. R.; Redondo, A.; Srinivasan, S. J. Electrochem. SOC.1988, 135, 2209-2214. (b) Wilson, M. S.; Gottesfeld, S. J. Electrochem. SOC.1992, 139, L28-L30. (3) (a) Collman, J. P.; Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F. C. J . Electroonol. Chem. 1979, 101, 117-122. (b) Collman, J. P.; Denisevich, P.; Konai, y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. SOC.1980. 102. 6027-6036. (4) Du;and,'R. R., Jr.; Bencosme, C. S.;Collman, J. P.; Anson, F. C. J. Am. Chem. SOC.1983, 105, 2710-2718. ( 5 ) Chang, C. K.; Liu, H.-Y.; Abdalmuhdi, I. J. Am. Chem. SOC.1984, 106 - - - , 2725-2726 - -- (6) Liu, H.-Y.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem. 1985, 89, 665-670. (7) Ni, C.-L.; Abdalmuhdi, I.; Chang, C. K.; Anson, F. C. J. Phys. Chem. 1987, 91, 1158-1166. (8) Le Mest, Y.; L'Her, M.; Courtot-Coupez, J.; Collman, J. P.; Evitt, E. R.; Bencosme, C. S. J . Electroonal. Chem. 1985, 184, 331-346. (9) Ngameni, E.; Le Mest, Y.; L'Her, M.; Collman, J. P.; Hendricks, N. H.; Kim, K. J. Electroanol. Chem. 1987, 220, 247-257. (IO) Le Mest, Y.; L'Her, M.; Collman, J. P.; Kim, K.; Hendricks, N. H.; Helm, S. J . Electroanal. Chem. 1987, 234, 277-295. (11) Collman, J. P.; Hendricks, N. H.;Leidner, C. R.; Ngameni, E.; L'Her, M. Inorg. Chem. 1988, 27, 387-393.
A B C Figure 2. Dioxygen binding modes for Co2(FTF4) proposed by Kim et
binding constant for dioxygen is lo3,whereas the binding constant for C O ~ ~ I / ~ I ( F is T Fimmeasurably ~) sma11.I3J4 More detailed studies of co2(FTF4) adsorbed on E X 1suggested that the first surface oxidation of FTF4) produces adsorbed [ C O ~ ' / ~ I ~ ( F T F ~ )Because ] X ? this oxidation occurs at the potential at which four-electron dioxygen reduction begins, the mixed-valence complex may be the active catalyst. Through the use of more rigorously purified Co2(FTF4)adsorbed on EPG, it has been shown that a change in the product distribution (water vs hydrogen peroxide) occurring between 0.5 and 0.3 V (vs NHE)coincides with the C0211/11-C0~1/111 redox couple. I I Thus present evidence suggests that the active form of the catalyst is the mixed-valence complex [ C O ~ ~ I / ~ ~ ~ ( F T F ~ ) ] X . In addition to these electrochemical studies of adsorbed and dissolved catalyst, structural studies of isolable complexes have been carried out in an attempt to establish a structural basis for the four-electron reduction mechanism. Although the crystal structure of Co2(FTF4) is known,I5 the structure of its bridged superoxo complex [(p-02)Co2(FTF4)]Xhas not been determined. From the Co-Co separation in Co2(FTF4) several orientations (12) (a) The site of the first oxidation of Co2(DPB) and Co2(FTF4) is highly dependent on reaction conditions. The first oxidation of the dicobalt diporphyrins dissolved in a noncoordinating solvent, e.g., dichloromethane, likely occurs on the ligand rather than on one of the metal centers. In the presence of dioxygen or other ligands, it is a cobalt center that is oxidized. (b) Le Mest, Y. Ph. D. Thesis, Universit6 de Bretagne Occidentale, Brest, France, 1988. (c) Ngameni, E.; LaouEnan, A.; L'Her, M.; Hinnen, C.; Hendricks, N. H.; Collman, J. P. J. Electrmnol. Chem. 1991, 301, 207-226. (1 3) Le Mest, Y.; L'Her, M.; Courtot-Coupez, J.; Collman, J. P.; Evitt, E. R.; Bencosme, C. S. J. Chem. SOC.,Chem. Commun. 1983, 1286-1287. (14) Le Mest, Y.; L'Her, M.; Collman, J. P.; Hcndricks, N. H.;McEIwee-White, L. J. Am. Chem. SOC.1986, 108, 533-535. (15) Kim, K.; Collman, J. P.; Ibers, J. A. J. Am. Chem. Soc. 1988,110, 4242-4246.
J . Am. Chem. SOC.,Vol. 114, No. 25, 1992 9811
Superoxo Complex of a Dicobalt Cofacial Diporphyrin
I 2.2
1.8
I
t
n
3
O2
3 4 7 2 ) 7639 775 no. of variables 0.071 NF)(F,2 > 34F,2)) error in observation of unit wt, e 2.04 “The low-temperature system is from a design by Prof. J. J. Bonnet and S. Askenazy and is commercially available from Sotorem, Z. I. deVic, 31320 Castanet-Tolosan, France. *The numbers in parentheses are the distances in mm between Friedel pairs of the preceding form. X-ray Crystallography. Single crystals of Co,(DPB) were obtained by vapor-diffusion of methanol into a chloroform solution at 5 “C. A red crystal of the substance was mounted in the cold stream (-120 “C) of an Enraf-Nonius CAD4 diffractometer. Unit cell parameters were determined by least-squares refinement of 25 reflections. Crystallographic details are given in Table 111. Data collection was performed at -120 OC. No systematic change was observed in the intensities of six standard reflections that were measured every 3 h of X-ray exposure. Intensity data were collected by previously described proced~res.’~ The structure was solved by a combination of Patterson and direct methods.’I An analytical absorption correction was made. Idealized hydrogen atom positions (C-H = 0.95 A, H C - H = 109.5O) were either calculated (non-methyl) or obtained from a least-squares adjustment of the observed positions (methyl) and were added as fixed contributions to the structure factors. The final anisotropic refinement on F, involved 7639 observations and 775 variables. The final positional and thermal parameters (Table SII) and structure amplitudes (Table SIII) are available as supplementary material. Preparation of Compounds. 2-Methyl-6-nitroacetaPnilide (1). This ~ 1.2 mol of compound was prepared as described in the l i t e r a t ~ r e ’using 2-methylaniline. The cream-colored product that precipitated upon addition of the reaction mixture to water was collected by vacuum filtration, washed with ice water, and partially dried by suction. The product was isolated by stirring the combined crude products of two nitrations in 9.33 L of Witt-Uterman solution5’ (7 L of H 2 0 , 0.78 kg of KOH,and 1.55 L of 95% ethanol) for 45 min. The resulting solution was vacuum-filtered to remove the unwanted 2-methyl-4-nitro(50) Sabat, M.; Ibers, J. A. J . Am. Chem. SOC. 1982, 104, 3715-3721. (51) The direct methods program used was DIRDIF. Beurskens, P. T.; Bosman, W. P.; Doesburg, H. M.; Gould, R. 0.;van den Hark, Th., E. M.; Prick, P. A. J. In Computational Crystallography; Sayre, D., Ed.; Clarendon: Oxford, 1982; p 516.~ (52) Howard, J. C. Organic Syntheses Coli. Vol. ZV; Wiley: New York, 1963; p 42-45. (53) Wepster, B. M.; Verkade, P. E. Rec. Trau. Chim. 1949,68,17-81.
9816 J. Am. Chem. SOC.,Vol. 114, No. 25, 1992 acetanilide isomer, The orange-red filtrate was acidified with 932 mL of glacial acetic acid. The precipitated product was collected by filtration, washed thoroughly with water, and dried in a vacuum oven at 60 OC to yield 542.1 g (60%) of yellow powder. The IH N M R spectrum is the same as that described in the l i t e r a t ~ r e . ’ ~ 2-Iodo-3-methylnitrobenzene (2).35In a 6-L reactor equipped with two reflux condensers and a mechanical stirrer were introduced 1 (542.1 g, 2.79 mol) and concentrated HCI (2.7 L). The suspension was heated carefully (to avoid foaming) with stirring to dissolve the yellow powder. The resulting orange solution became deep orange after 7 h at reflux. The solution was cooled to 5-10 OC with an ice-water bath. An orange precipitate formed. A solution of N a N 0 2 (241.1 g, 3.49 mol) in 875 mL of H 2 0was added dropwise from a dropping funnel slowly enough so that the temperature remained between 8 and 10 OC. When the addition was complete, the solution was stirred for an additional 30 min and poured slowly into a stirred solution of KI (926 g, 5.58 mol) in 3.5 L of ice water. During the addition, yellow gas (nitrogen and iodine) was evolved and a thick, dark-brown foam developed. At the end of the addition, NaHSO, (aqueous solution, d = 1.24, ca. 165 mL) was added until the iodine color dissipated. The peach-colored product was recovered by filtration, washed thoroughly with water, and dried under vacuum to yield 715.2 g (97.4%). The ‘ H N M R spectrum is the same as that described in the literat~re.,~ 6,6’-Dimethyl-52’-diNtrobiphenyl (3).35Under a nitrogen flow, a 6-L (2)(71 5.2 g, 2.72 reactor was charged with 2-iodo-3-methylnitrobenzene mol), dry DMF (distilled and dried over 4-A molecular sieves, 3 L), and Cu powder (716 9 ) . The mixture was heated to reflux with stirring. After 4 h an additional portion of Cu (716 g) was added and the reflux was continued for 2 h. The hot mixture was vacuum filtered through Celite (200 g). The inorganic material was washed with ether (2 X 1.1 L) in order to remove all the organic product. Ether was removed from the washings by rotoevaporation. The remaining residue and initial filtrate were combined and poured into 22 L of water. The resulting milky precipitate coagulated on standing. After filtration and partial drying by suction, the grey solid was recrystallized from 4 L of 95% ethanol to give light orange crystals of 3. The mother liquor was concentrated to provide more product, total yield, 322 g (87%). The ‘H N M R spectrum is the same as that described in the l i t e r a t ~ r e . ~ ~ 1,lO-Dimethylbenzo(c]cinnoline(4).35 A 20-L reactor equipped with a large reflux condenser, mechanical stirrer, dropping funnel, and N 2 inlet was charged with fresh LiAIH4 (217.5 g, 5.73 mol) and anhydrous ether (4.88 L). A solution of 6,6’-dimethyl-2,2’-dinitrobiphenyl(3) (322 g, 1.18 mol) in dry benzene (4.88 L) was added dropwise so as to maintain a gentle reflux. The addition was complete after ca. 4 h. Decomposition of e x m s LiAlH4 was achieved by the careful addition of water (220 mL), 15% aqueous N a O H (220 mL), and water (660 mL). The granular inorganic residue was removed by gravity filtration. Air was bubbled through the stirred orange filtrate in order to ensure complete oxidation of the hydrazo group. The solvent was rotoevaporated to provide pure 4 as a brown-yellow solid (241.1 g, 97.9%). The ‘ H N M R spectrum is the same as that reported in the I i t e r a t ~ r e . , ~ 1,8-Dimethylbiphenylene(5).35The pyrolysis of 4 was performed in a quartz tube in a horizontal tube furnace heated to 800 OC. The system was evacuated with a mechanical vacuum pump. 1,lo-Dimethylbenzo[clcinnoline (4, 6.24 g) was placed in a 100” two-necked round-bottomed flask along with a magnetic stirbar. One neck of the flask was connected to the quartz pyrolysis tube by a vacuum distillation adapter wrapped with heating tape. The other neck was fitted with a groundglass stopper. The receiving end of the quartz tube was connected to a 100” two-necked round-bottomed flask (cooled with liquid N 2 during the pyrolysis) with another vacuum distillation adapter. The vacuum was applied to the second neck of the collection flask. Two liquid N,-cooled traps were placed between the receiving flask and the vacuum pump. The oven was heated to 800 OC, the apparatus was evacuated, and the heating tape was set to 195 OC. The solid 4 was stirred and heated with an oil bath. The pyrolysis commenced when the oil bath reached about 170 OC. The oil bath temperature stabilized at 200 OC about 10 min after the reaction had begun. After an additional 35 min, the oil bath was removed, and the system was vented in order to add more 4 for another pyrolysis. The pyrolysates from eight 6.24-g samples (total 49.92 g, 0.24 mol) of 4 were recovered from the pyrolysis apparatus with dichloromethane. The dark solid obtained upon removal of the solvent (41.4 g) was dissolved in heptane and purified by flash column chromatography (10- X 5-cm. S O 2 , heptane). The column was eluted with heptane until all of the desired product had been removed. The heptane was removed in vacuo. Recrystallization from 74 mL of methanol (-20 OC) yielded pale yellow flakes (20.56 g, 47.5%). ‘H N M R (400 MHz, CDCI,, ppm): ~~
(54) Walker, F. A,; Beroiz, D.; Kadish, K. M. J . Am. Chem. Sot. 1976, 98, 3484-3489.
Collman et al. CH, 2.18 (s, 6 H); 2,7- and 4,5-biphenylene 6.46 (d, 2 H ) and 6.53 (d, 2 H), 3.6-biphenylene 6.63 (dd, 2 H). 1,8-Bis(bromomethyl)biphenylene (6). A mixture of 1,8-dimethylbiphenylene (5)(10.28 g, 57 mmol), N-bromosuccinimide (22.6 g, 127 mmol),benzoyl peroxide (0.5 g), and carbon tetrachloride (250 mL) was stirred and refluxed for 5 h. Additional benzoyl peroxide (0.5 g) was added each hour. Insoluble material was removed from the hot mixture by gravity filtration, and most of the solvent was evaporated. Heptane was added to induce precipitation. The precipitate was collected by filtration on a frit, washed with heptane, and dried. A first recrystallization from 120 mL of chloroform (-20 “C) followed by concentration of the filtrate to yield more crystals gave 6 as beige needles (8.24 g, 42.7%). ‘H N M R (400 MHz, CDCI,, ppm): CH2Br 4.43 (9, 4 H), H-biphenylene 6.60 (d, 2 H ) and 6.75 (m,4 H). 1,8-Diformylbiphenylene(7). 1,8-Bis(bromomethyI)biphenylene(6) (10 g, 29.6 mmol) and tetra-n-butylammonium dichromate (82.94 g, 118 mmol) were dissolved in CHCI, (430 mL) and refluxed for 3 h. The cold mixture was filtered through a silica gel pad. The silica was washed with ether until no more product was eluted (as monitored by TLC, S O 2 , dichloromethane, I?, = 0.45). The solvent was removed in vacuo, and the crude material was purified by flash chromatography (5- X 25-cm, S O 2 , dichloromethane) to give 4.35 g (70.6%) of bright yellow powder. ’ H N M R (400 MHz, CDCI,, ppm): C H O 10.32 (s, 2 H), 2,7-biphenylene 7.27 (d, 2 H), 3,6-biphenylene 7.23 (dd, 2 H), 4,5-biphenylene 6.86 (d, 2 H).
1,8-Bis(5,5’-bis( benzyloxycarbonyl)-4,4’-diethyl-3,3’-dimethyl-2,2’dipyrrylmethyl)biphenylene (10). 1,8-Diformylbiphenylene (7) (6.25 g, 30 mmol) and 2-(benzyloxycarbonyl)-3-ethyl-4-methylpyrrole(8)37-41 (29.27 g, 120 mmol) were dissolved in absolute ethanol (300 mL, degassed by bubbling with argon for 20 min) that contained concentrated HCI (5.9 mL). The solution was stirred and heated at reflux under argon for 3.5 h. It was then allowed to cool with stirring to room temperature and was next placed in a freezer. The resultant brown-red precipitate was filtered, washed with cold methanol, and dried (31.26 g, 91%). IH N M R (400 MHz, CDCI,, ppm): N H 8.28 (br s, 4 H), phenyl-H 7.33 (m, 20 H), 3,6-biphenylene 6.70 (t, 2 H), 4,5-biphenylene 6.64 (d, 2 H), 2,7-bipenylene 6.05 (d, 2 H), CH2Ph 5.25 (d, 8 H), C H 4.76 (s, 2 H), CHZCH, 2.66 (q, 8 H), CH3 1.44 (s, 12 H), CH2CH3 1.02 (t, 12 H). 1,8-Bis(4,4’-diethyl-3,3’-dimethyC52’-~p~~~yl)bip~ylene (12). The hydrogenolysis of the tetrabenzyl ester 10 (61.32 g, 53 mmol) was carried out in a 3-L filter flask containing T H F (2.2 L), triethylamine (24.3 mL), and 10% Pd/C (12.15 g) under H 2 (1 atm). The consumption of H 2 stopped after 2 h. The mixture was stirred under H 2 for an additional 3 h and filtered through Celite to remove the catalyst. The Celite was washed with ethanol until only catalyst remained. The combined filtrate and washings were rotoevaporated to yield 1,8-bis(5,5’dicarboxy-4,4’-diethyl-3,3’-dimethyl-2,2’-dipyrrylmethyl) biphenylene (11) as a pale yellow powder. The tetracarboxylic acid 11 was taken up in 600 mL of ethanolamine and heated to reflux under argon for 1.5 h. The hot solution was poured with stirring into 6 L of ice water and allowed to stand for 30 min. The yellow precipitate was collected by vacuum filtration, washed with water, and dried under vacuum over P205(32.3 g, 99%). The yellow solid 12 was stored under an inert atmosphere at -18 OC. ‘ H N M R (400 MHz, CDCI,, ppm): N H 7.35 (br s, 4 H), 3,6-biphenylene 6.66 (t. 2 H), 2,7or 4,s-biphenylene 6.55 (d 2 H), 5-pyrrole 6.34 (s, 4 H), 2,7- or 4,5biphenylene 6.26 (d, 2 H), C H 4.93 (s, 2 H), CH2CH, 2.39 (q, 8 H), CH3 1.61 (s, 12 H), CHZCH, 1.16 (t, 12 H). 1,8-Bi@-( 2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene (H,(DPB)). Under argon, a suspension of 12 (4.88 g, 8 mmol) and 5,5’-bis(benzyloxycarbonyl)-4,4’-diethyl-3,3’-dimethyl-2,2’-dipyrrylmethane (9)42,43$45*46 (4.82 g, 16.8 mmol) in dry methanol (1.3 L) was stirred for 1 h. A solution of p-toluenesulfonic acid (3.56 g) in methanol (20 mL) was added dropwise to the stirred suspension via a cannula over a 2-h period. The resulting dark-red solution was stirred in the dark for 48 h. To the solution was added o-chloranil (tetrachloro-l,2-benzoquinone,3.56 g), and the stirring was continued for 3 h. A saturated, aqueous N a H C 0 3 solution (1.2 L) was added to precipitate the organic products. The precipitate was collected by filtration through Celite. It was then washed with water until the pH of the water was 7, and it was next dissolved in dichloromethane. The solution was washed with brine, and the volume was reduced to ca. 600 mL. A saturated solution of zinc acetate in methanol (60 mL) was added, and the solution was heated at reflux for 1 h. After the solvent was removed, the residue was taken up in dichloromethane and chromatographed on silica gel (7.5- X 17-cm). The first band eluted by dichloromethane was collected. The eluent was concentrated (to ca. 200 mL) and vigorously stirred with 6 N HCI (16 mL) for 15 min. The solution was neutralized with 10% aqueous sodium carbonate (36 mL), and the mixture was stirred for 15 min. The organic phase of this mixture was separated,
J . Am. Chem. SOC.1992, 114, 9877-9889 washed with water, dried (MgSOJ, and evaporated. The residue was redissolved in dichloromethane and flash chromatographed on silica gel (230-400 mesh, 5- X 25-cm). A band eluted first with 5% MeOHCH2CI2solution. A second fraction was collected with 10-20% MeOHCH2C12solution. The product was eluted with 20-70% MeOHCH2CI2 solution. Partial evaporation of the solvent removed mainly dichloromethane to yield pure H4(DPB) as purple microcrystalline powder (2.0 g, 23%). The IH NMR spectrum is the same as that reported in the literature.24 Co,(DPB). In a dinitrogen drybox, a solution of 107 mg of free-base DPB, 364 mg of anhydrous cobalt(I1) chloride, and 15 drops of 2,6lutidine in 125 mL of THF was stirred and refluxed for 20 h. The resulting mixture was filtered through a column of neutral, activity I alumina (1- X 15-cm)to remove unreacted cobalt chloride. The solvent was removed under vacuum to yield Co,(DPB) as a blood-red powder. Crystallization from dichloromethane/methanol yields analytically pure CO,(DPB). Anal. Calcd for C76H76NsC02: C, 74.86; H, 6.28; N, 9.19. Found: C, 74.83; H, 6.05; N, 9.04. Crystals for X-ray analysis were grown by slowly diffusing methanol into a chloroform solution of Co2(DPB) at 5 "C. UV/vis (dichloromethane, nm): 382 (Soret), 530, 559 (lit.Io 382, 530, 558'nm). [Co,(DPB)XPF,]. In a dinitrogen drybox, 19.5 mg of Co2(DPB)was dissolved in 15 mL of benzene with vigorous stirring. A THF solution of AgPF6 (360 pL of a 4.31 X M solution (21.8 mg in 2 mL of THF)) was added dropwise to the stirred solution. The mixture was stirred for 4 h after the final addition of oxidant. The reaction mixture was filtered through a glass-fiber plug in a pipette to remove precipitated silver metal and the oxidized product. The plug was washed with several milliliters of benzene to remove any unreacted starting material. The
9877
product was collected by washing the plug with dichloromethane until the washings remained colorless. Subsequent removal of the solvent under reduced pressure yielded 20.3 mg (93%) of [Co,(DPB)] [PF,]. UV/vis (dichloromethane,nm): 372 (Soret), 524, 552 (lit.Io 372 (Soret), 540 nm). [(C(.~~)CO~(DPB)(DPI~)~IPF,]. 1,5-Diphenylimidazole(360 pL of an 8.33 X M solution in dichloromethane) was added to a dioxygen-saturated solution of 20.3 mg of [Co2(DPB)][PF,]. The solvent was evaporated away under a stream of dioxygen. Recrystallizationfrom chloroform/methanol yielded analytically pure [(p-O,)Co,(DPB)C,~69.31; ~ O ~H, P: (DPIm)2[PF6]. Anal. Calcd for C ~ O ~ H I & O ~ F ~ N 5.49; N, 9.15. Found: C, 69.43; H, 5.31; N, 9.08. UV/vis (dichloromethane, nm): 334, 410 (Soret), 534, 568. MS: LSIMS (tetraglyme) 1692 (cluster) [M+ - PF6]. Acknowledgment. W e thank Kimoon Kim and Michele McMahon for contributions to the development of the improved synthesis of the free-base diporphyrin. W e thank the National Science Foundation, the National Institutes of Health (Grants GM-17880 to J.P.C. and HL-13157 to J.A.I.), and the CNRS for financial support. Supplementary Material Available: Table SI, additional bond distances and angles for Co2(DPB), Table SII, final positional and thermal parameters for Co2(DPB), and improved, large scale p r d u r e - s for the preparation of compounds 8 and 9 (19 pages); Table SIII, structural amplitudes for Co,(DPB) (31 pages). Ordering information is given on any current masthead page.
Synthesis and Characterization of Novel Cobalt Aluminum Cofacial Porphyrins. First Crystal and Molecular Structure of a Heterobimetallic Biphenylene Pillared Cofacial Diporphyrin Roger Guilard,*JPMichel Angel Lopez,ln Alain Tabard,'" Philippe Richard,'" Claude Lecomte,lbSt6phane Brandes,lPJames E. Hutchison,lc and James P. CoUmanlC Contribution from the Laboratoire de Synthgse et d'Electrosynth2se Organomttalliques, Associt au C.N.R.S. (URA 33), Facultt des Sciences "Gabriel", Universitt de Bourgogne (Dijon), 6, boulevard Gabriel, 21 100 Dijon, France, Laboratoire de Mintralogie et Cristallographie, Associt au C.N.R.S. (URA 8091, Universitt de Nancy I, B.P. 239, 54506 Vandoeuvre les Nancy, France, and Department of Chemistry, Stanford University, Stanford, California 94305. Received April 8, 1992
Abstract: The synthesis of the novel family of heterodinuclear complexes (DP)CoAl(OR) (where D P - is the tetraanion of the diporphyrin biphenylene DPB or the diporphyrin anthracene DPA, and R = CH3, CH2CH,, or CH2C,H5) is reported. These complexes were obtained by selective metalation of the cofacial diporphyrins with cobalt and aluminum. Each (DP)CoAI(OR) complex was characterized by mass spectrometry and UV-vis, IR, ESR, and IH NMR spectroscopies. Unusually large paramagnetic shifts were observed for the protons on axial ligands bound to aluminum and for the porphyrinic N-H protons of the monocobalt DPB and DPA complexes. Analysis of the paramagnetic shifts indicates that the main contribution to isotropic shifts arises from a through space (or dipolar) interaction of an unpaired electron on cobalt(I1). Structural data were deduced from the IH NMR study. In addition, the molecular structure of the cobalt(I1) aluminum(II1) ethoxide diporphyrin biphenylene (DPB)CoAI(OCH2CH3)was determined by X-ray diffraction. This is the first crystal structure of a heterobimetallic cofacial diporphyrin. (DPB)CoA1(OCH2CH3)( C ~ ~ H ~ I N ~ O C O )Acrystallizes ~ * C ~ H in the triclinic system, space group Pi. Its lattice constants are as follows: (I = 13.095 (4) A, b = 16.836 (3) c = 16.986 (3) A, CY = 87.47 (l)', j3 = 70.40 (3)O, y = 85.90 (2)O, V = 3516 A3, Z = 2, R(F) = 5.30%, R,(F) = 5.27%, GOF = 2.5 for 6800 reflections with I t 3 4 0 . No metal-metal interaction occurs (Co-AI = 4.370 (1) A), and both porphyrin moieties are slipped by CY = 29.8O. Finally, the tedious synthesis of the (DPA)H4free-base porphyrin has been extensively modified, and several new reaction steps are presented leading to significant increases in both scale and yield.
1,
Many efforts of our groups have been devoted to the study of metalloporphyrins with metal-metal bonds2-12in order to inves-
tigate the major factors which affect the metal-metal interact i o n ~ . ~ In , ~these ~ ~ ~compounds most of metal-metal bonds
( 1 ) (a) Universit€de Bourgogne (Dijon), (b) Universitt de Nancy I. (c) Stanford University.
(2) Bark, J.-M.; Guilard, R.; Lecomte,C.; Gerardin, R. Poryhedron 1984, 3, 889-894.
0002-7863/92/1514-9877$03.00/0
0 1992 American Chemical Society
